Review articleImpedance spectroscopy: Over 35 years of electrochemical sensor optimization
Introduction
Significant technological advances over the past few decades have led to the development of numerous analytical devices in the monitoring of a wide range of analytes. The desire to measure and understand every single variable in our environment has been the impetus behind the growth of many innovative tools. More specifically, electrochemical-based sensors are one group of analytical devices that have attracted considerable attention in recent times. The introduction of electrochemical sensors in the last century has revolutionized the way in which we lead our lives. This is not surprising considering that they play a crucial role in medical and clinical analysis, environmental and industrial monitoring [1], [2], [3], [4], [5], [6]. The common feature binding all electrochemical sensors is that they rely on the detection of an electrical property (i.e., potential, resistance, current), and are normally classed according to the mode of measurement (i.e., potentiometric, conductometric, amperometric). A number of excellent reviews appear on the topic [7], [8], and the authors recommend them for further reading. Indeed, electrochemical sensors is a large field that continues to evolve, and this is partly due to the fact that the underlying electrochemical principles for the detection of analytes are highly relevant in many diverse areas.
A fundamental challenge pertinent to electrochemical sensor design lies in the molecular understanding of the relationship between surface structure and reactivity. The fabrication of sensor materials with unique response characteristics has created a pressing need to understand their chemical and physical properties. Understanding the fundamental processes that govern sensor response in most cases leads to the development of electroanalytical devices with superior selectivity, excellent chemical stability, higher sensitivity, and lower detection limits. In order to achieve these objectives, it has been necessary to study various electrical processes that occur at the surface of the sensor or inside the sensor membrane itself. This may involve tailoring of the chemistry of the surface layer so that it exclusively adsorbs the target molecule/ion in the presence of interferences, or it could involve altering the bulk electrical conduction properties of the sensor by modifying the membrane composition. Sensor optimization is one of the most crucial steps in the realization of an electroanalytical device. Whether it involves tinkering with the interfacial properties or modifying the membrane composition, a suitable tool that allows the sensor performance to be evaluated under a range of conditions must be employed. Understanding how various parameters influence the response mechanism and interfacial reaction kinetics will assist with the development of electrochemical sensors with new and improved response characteristics.
There are countless techniques available for electrochemical sensor development/optimization (i.e., cyclic voltammetry, scanning electrochemical microscopy, etc.) [9], [10], [11], and it is beyond the scope of this review to discuss the relative merits of each technique. A majority of these methods probe the membrane/electrolyte interface by using a large perturbation, which is designed to provide mechanistic information by driving the reaction to a condition far from equilibrium. Another approach, however, is to apply a small perturbation to ensure that the kinetic information pertaining to the membrane/electrolyte interface is at near zero current conditions. Subsequently, electrochemical impedance spectroscopy (EIS) is a non-destructive steady-state technique that is capable of probing the relaxation phenomena over a range of frequencies [12]. The power of EIS lies in its ability to provide in situ information on relaxation times over the frequency range 106 to 10−4 Hz. It is a tool that has been used to identify and separate different contributions to the electric and dielectric responses of a material. Traditionally, EIS has provided a wealth of information pertaining to the corrosion rate and corrosion processes on a wide variety of metals and metal-coated surfaces [13], [14]. Nowadays, we are finding an increasing use of EIS to investigate the adsorption and charge transfer processes of many types of electrochemical sensors.
The purpose of this article is to review recent advances and applications of EIS in unravelling the response mechanisms of electrochemical-based sensors. Consequently, the authors will evaluate the effectiveness of EIS as a technique for investigating sensor response processes, and have aimed the article at a specialized electrochemistry readership. Ultimately, the review focuses on the use of EIS in the development of electrochemical-based sensors and biosensing devices.
If a novice to the EIS technique finds themselves interested in the subject matter of this fundamental review then the authors strongly recommend the superb EIS text by Macdonald [12], along with a compilation of excellent research articles [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], to obtain a good grounding in the fundamental principles underpinning the EIS technique, as this rudimentary information is beyond the scope of this specialized review paper.
Section snippets
Potentiometric sensors
Also known as ion-selective electrodes (ISEs), these sensors encompass a large subset of electrochemical sensors. In many cases, the potentiometric sensor comprises a membrane with a unique composition, noting that the membrane can be either a solid (i.e., glass, inorganic crystal) or a plasticized polymer, and the ISE composition is chosen in order to impart a potential that is primarily associated with the ion of interest via a selective binding process at the membrane–electrolyte interface.
Electrochemical biosensors
Biosensors have received a lot of attention lately, and this is not surprising considering that they play a significant role in the monitoring of a wide range of diseases and pathogens. The diagnoses and management of the worldwide health problem of diabetes has made life much easier for many patients since the development of the electrochemical-based glucose biosensor [128]. A biosensor is an analytical device, which incorporates a biological recognition element in close proximity or
Future developments
EIS characterization relies on a system whose electrical behaviour is dependent on various processes, which are responsive at different rates or frequencies. It is well known that EIS measurements normally take a long time, i.e., it can take from anywhere between ∼10 min to more than several hours to collect an EIS spectrum. Obviously, this will depend on the relaxation processes/stability of the system under study, and the frequency range chosen. Unfortunately, this can lead to interpretation
Conclusions
The increasing use of electrochemical sensors in medical, industrial and environmental applications has created a pressing need to understand the bulk and surface properties of such important analytical systems. It appears that the scientific community has embraced impedance spectroscopy for characterizing a wide range of electrochemical sensor systems. The mechanistic information gathered from this technique has been used to fabricate electrochemical sensors with desirable properties. Indeed,
Acknowledgements
The financial support of the Australian Research Council (ARC) and the Australian Biosecurity Cooperative Research Centre (AB-CRC) are gratefully acknowledged.
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